The present invention relates to a device for determining and following the instantaneous coordinates of the extreme value of a curve y=f(x), characteristic of a system having an input x and an output y.
This device can in particular be used for obtaining information on the electron temperature and electron density of the ionospheric plasma or the magnetospheric plasma.
Within the scope of this application, the system having an input x and an output y is constituted by a high frequency sensor supplying an electrical signal. The electrical signal, in the case of a sensor suitable for measuring the electron density of the ionospheric plasma can, for example, be illustrated in the manner shown in FIG. 1 by a curve giving the variations of the impedence z/z.sub.0 of the sensor as a function of the frequency f, expressed in megahertz. As can be seen, this curve has a main maximum M. Through knowing the coordinates x.sub.M, y.sub.M of this maximum and in particular the abscissa x.sub.M (frequency), information can directly be obtained on the electron density, N, of the ionospheric plasma. Since the plasma frequency, .OMEGA.=Ne.sup.2 /m.epsilon..sub.u, the electron density is easily calculated once the plasma frequency is known. Depending on the type of sensor used, the plasma frequency is equal to the maximum value from the sensor either directly or by way of a simple calculation. This determination of the coordinates of the maximum takes place by means of an electronic circuit directly connected to the input x and the output y of the sensor and which processes the signal supplied by the latter.
In general, the determination of coordinates of the extreme value, i.e. the maximum or minimum of a curve y=f(x) takes place with the aid of a device, which scans the x's. Unfortunately this type of determination leads to the scanning of zones which are without interest for obtaining information on the coordinates of the extreme value and this leads to a considerable time loss. This can be a nuisance in the case where the extreme value of the curve y=f(x) moves over a period of time. This is almost always the case, when this curve represents physical quantities such as, for example, the electrical signal supplied by a sensor used for determining the electron temperature or density of the ionosphere.
In the case of an evolution over a period of time of the extreme value of curve y=f(x), the devices using a scan of the x's, in view of their response time, do not make it possible in many cases to determine the coordinates of the extreme value with a good level of accuracy and do not make it possible to follow the evolution of the latter over a period of time.
In order to obviate these disadvantages, the use has been envisaged of devices limiting the scan of the x's to the values close to those of the abscissa x.sub.M of the extreme value. These devices are generally called extremely servocontrol devices. The description and operation of these devices appear in a book published by DUNOD in 1976 and entitled "Introduction aux systemes asservis extremaux et adaptatifs" and whose authors are P. DECAULNE, J. CH. GILLE and M. PELEGRIN.
In this type of device, the determination of the coordinates x.sub.M, y.sub.M of the extreme value of a curve y=f(x) takes place by choosing a point on the coordinates x.sub.0, y.sub.0 close to the extreme value and by modifying the control value x and consequently y up to the time when x-x.sub.M =0 is obtained. This is in fact a servocontrol process.
FIG. 2 shows a gradient-type extreme servocontrol device operating on the basis of this principle. This device comprises a divider 2 having two inputs and an output, a first differentiator 4, whose input is connected to the input x of a system 6 (sensor) and whose output is connected to one of the inputs of the divider 2. There is also a second differentiator 8, whose input is connected to the output y of system 6 and whose output is connected to the other input of divider 2. There is also an integrator 10, whose input is connected to the output of divider 2 and whose output is connected to the input x of system 6.
Although they provide the extreme value coordinates more rapidly than the previously described devices and although they make it possible to follow the evolution of the extreme value over a period of time, the response time of such a device is much too long, when the extreme value of the curve moves at a high speed.
In addition, the response of this device, i.e. the variations of x as a function of time, starting from a value x.sub.0, it only makes it possible to reach the value x.sub.M in an asymptotic manner, which is illustrated by the curve of FIG. 3a.
In view of the fact that x.sub.M is reached asymptotically, this device only makes it possible to obtain the value of abscissa x.sub.M of the extreme with a very good precision at the end of an infinite time. Conversely, the value of the abscissa x.sub.M of the extremum can be obtained at the end of a very short time, but with a poor precision.